1. Technical Field
The present invention relates generally to bus circuits, and more particularly to an MTCMOS bus circuit and operating method for reducing power consumption by a bus circuit.
2. Description of the Related Art
Low power electronic systems incorporating large parallel buses are increasingly prevalent, as microprocessor systems are used in notebook computers, personal digital assistants (PDAs) and other electronic appliances designed for portable battery-operated use.
A large portion of the quiescent power consumption of such systems in “standby” or other low-power modes is now determined by the quiescent “off-state” leakage of the bus repeater devices and gate tunneling at the input of the devices, as the repeaters are used in increasing quantities. The increased number of repeaters are needed to reduce the effects of increased wire capacitance and resistance as bus frequencies are increased and as the inter-wire spacing is reduced due to device scaling.
Therefore, as the number of bus repeaters increases, the percentage of standby power consumption due to the above-mentioned leakage is increasing, and is becoming the dominant portion of the total system standby power consumption.
One architecture that has been implemented for reducing quiescent power consumption in buses is Multi-Threshold Complementary Metal-Oxide Semiconductor (MTCMOS) architecture. The MTCMOS architecture isolates the power supply rails of the CMOS bus repeaters by using higher threshold voltage devices as power rail “header” and “footer” standby switching devices feeding common internal “virtual supply” rails. The use of higher threshold voltage devices decreases the off-state leakage and gate tunneling effects that otherwise increase power consumption when the repeaters are not switching. However, an area penalty is generated by the additional devices, so for practical purposes, many repeaters must share a common set of standby switching devices. Also, the virtual supply rails must be fairly short (and thus the devices connected to the virtual supply rails must be fairly local), or a penalty is incurred in routing the virtual supply rails—especially if scarce upper layer metal resources are used.
As the bus repeaters are typically disposed between parallel bus segments separated by the repeaters), localized groupings of the repeaters are generally a group of repeaters that drive a given group of bus segments, one for each of the bitlines, it is generally more practical to share the standby switching devices among a given set of repeaters for each bit line. However, the standby signal that controls the standby switching devices must be routed to each of the switching devices and the delay time for recovery from the standby state (i.e., the time needed for the virtual power supply rails to stabilize after the standby switching devices are turned on) typically limits the use of the standby mode to coarse intervals. Thus, the standby mode is entered only when a long interval of inactivity is expected, limiting the amount of power reduction that can be accomplished. Further, due to the use of the virtual power supply rails across the set of bitlines, the virtual power supply rails must have sufficient capacitance to handle worst-case switching loads for each repeater on the bus combined, increasing the delay penalty associated with leaving the standby mode.
Further, if finer control of the standby mode of an MTCMOS bus repeater were possible, a control mechanism would be needed to control the standby mode, not from the typical global or local block device power mode signal as is commonly used in battery-operated circuits, but a dynamic control mechanism that can control the standby mode in response to bus activity.
Therefore, it would be desirable to provide an MTCMOS architecture such that a finer interval may be used to enter and exit standby mode so that bus power consumption due to leakage can be reduced. It would further be desirable to provide a control mechanism such that the standby mode for a set of bus repeaters can be controlled on-the fly.